A security-centric operating system, which is believed to be the most secure OS currently available. OpenBSD was started by Theo De Raadt after disagreements between himself and members of another BSD team. As a side note, FreeBSD does have more mainstream support in general, and is quite similar to OpenBSD, but only OpenBSD makes overall system security its primary focus. Including strong cryptography directly into the system itself, as well as many, many other enhancements, makes OpenBSD an ideal operating system for sites which may be focused on by crackers.
Firewalls, VPN servers, and webservers are only a few areas in which OpenBSD shines due to its security-centric, crack-resistant system design.
It should be noted that the OpenBSD team also created
OpenSSH, a free implementation of the SSH (secure shell) protocol. (www.openssh.com)

Update: If you run OpenBSD, you may be interested in visiting the OpenBSD Metanode, a work in progress.. edit: which apparently has been deleted by the mods? ... sigh.